Maybe somebody who has a lot of a given bond has to pay for his daughter's wedding. So he puts the bonds on the market. There is suddenly more supply, so the price goes down. Obviously, the fundamentals haven't changed. It is all supply and demand. Of course, when there is bad news about a company, people are less likely to want to buy its stock or bonds, but as you have seen, price swings happen all the time in the absence of any such news.